The present invention relates to microdischarge devices and, in particular, to microdischarge devices used as light detectors.
It has long been known that electrical discharges are efficient sources of light, and today gas discharge lamps (including fluorescent sources, and metal-halide, sodium, or mercury arc lamps) account for most of the world""s light-generating capacity (several billion watts on a continuous basis). Most of these devices are, unfortunately, bulky and frequently have fragile quartz or glass envelopes and require expensive mounting fixtures.
Along the same vein, the photosensitivity of low pressure gas discharges over a broad region of the electromagnetic spectrum has been well known for several decades. Discharge-based microwave and millimeter wave detectors were first demonstrated in the 1950s, and, in 1969, Kaplafka reported the detection of CO2 laser radiation (10.6 xcexcm) radiation by a He discharge, attributing the effect to photoabsorption by the He diatomic excimer or the dimmer ion. Several groups have also exploited inverse bremsstrahlung to measure relative electron densities in a plasma through the absorption of infrared or far-infrared radiation. More recently, in the 1970s, Farhat, Kopeika and co-workers reported and characterized photodetection in the visible and ultraviolet (UV) with low pressure rare gas lamps. Despite the impressive responsivities measured in the near-UV (5-10 A/W at 300 nm) in their studies, the physical size of conventional gas-filled devices continues to be a severe drawback to their use as photodetectors.
However, with the recent reduction in size of discharge devices, as well as fabrication in other materials, many more possibilities exist for application of these devices. In one example, microdischarge devices may be fabricated in silicon by techniques developed in the integrated-circuit industry. FIG. 1 shows a conventional microdischarge device 100. The microdischarge device 100 is fabricated in silicon and has a cylindrical cavity 102 in the cathode 104 of the device 100. The Si wafer from which the cathode 104 is fabricated is affixed to a copper or diamond heat sink or thermoelectric cooler with conductive epoxy. The anode 106 for the microdischarge device 100 is typically a metal film such as Ni/Cr. A thin dielectric layer 108 deposited onto the silicon electrically insulates the cathode 104 from the anode 106. When the cavity 102 is filled with the desired gas and the appropriate voltage imposed between the cathode 104 and the anode 106, a discharge is ignited in the cavity 102.
Despite this reduction in size, until now microdischarge devices have been used solely as emission devices. The use of such devices and arrays of devices as detectors would thus lead to a host of applications in, for example, remote sensing, biomedical diagnostics, and spectroscopy. Thus, a photodetector that is highly sensitive compared with conventional photodetectors, relatively inexpensive to fabricate into arrays, readily able to be integrated with conventional electronic or optoelectronic devices and amenable to mass production would be of considerable value.
In light of these objectives, as well as other objectives discussed herein, a microdischarge photodetector, method of forming and using the photodector, and detection system using the photodetector are disclosed herein.
In one embodiment the microdischarge photodetector comprises a photocathode, an anode, an insulator disposed between the photocathode and anode, and a gas disposed in a cavity formed in the insulator. The gas has a breakdown voltage and impact ionization coefficient. When the gas breaks down into a plasma, the impact ionization coefficient is sufficient to cause avalanche breakdown when light of photon energy larger than about a work function of the photocathode is incident on the photocathode. The resulting photoelectrons are ejected from the photocathode.
The cavity may be tapered and may extend into the photocathode. The depth of the cavity in the photocathode may be at most about 60 xcexcm. An optically transmissive material may seal the cavity. A shape of cavity may be independent of material that forms the photocathode.
The photocathode may be a semiconductor. An underlying substrate may be formed from the same material as the photocathode or a different material as the photocathode. The underlying material may be substantially different from the photocathode. The thickness of the photocathode may be between about 1-5 absorption lengths of the peak absorption of the light to be absorbed. The photocathode may be coated with a material having a higher secondary electron emission coefficient or emission in a different wavelength range than the photocathode.
The insulator may comprise a plurality of dielectric layers with at least two of the plurality of dielectric layers having different dielectric constants.
The anode may comprise an electrically conducting screen.
The breakdown voltage of the gas may be at most about 120V so that the plasma may be formed at conventional wallplug voltages.
Arrays of photodetectors may comprise a first set of photodetectors and a second set of photodetectors that are electrically isolated from each other and thus operable independently from each other.
In another embodiment, a method of fabricating the microdischarge photodetector comprises forming the photocathode on the substrate, forming the cavity in the insulator disposed on the photocathode, forming the anode on the insulator, and introducing the gas into the cavity.
The method may further comprise limiting the area of the cavity at the surface of the photocathode to at most about (500 xcexcm)2, extending the cavity into the photocathode, limiting a depth of the cavity in the photocathode to about 60 xcexcm, sealing the cavity with an optically transmissive material, tapering the cavity in the photocathode, shaping the taper as determined by the lattice structure of the photocathode, or wet etching the photocathode to form the taper.
The method may further comprise forming the photocathode from a semiconductor, limiting the thickness of the photocathode to between about 1-5 absorption lengths for a specific wavelength associated with the incoming optical radiation as above, forming the photocathode on the substrate such that the photocathode contacts the substrate and forming the photocathode and substrate from different materials.
The method may further comprise forming the insulator from a plurality of dielectric layers, at least two of which have different dielectric constants, affixing a conducting screen to an end of the cavity, forming the anode from an electrically conducting screen, or limiting the breakdown voltage of the gas or operating voltage of the photodetector to at most about 120 V.
The method may further comprise arranging a plurality of photodetectors into an array of photodetectors and filling different cavities in the array with different gases or fabricating the photocathodes such that a first set of photocathodes and a second set of photocathodes are formed from different materials that have different work functions.
In another embodiment, a method of detecting light using a microdischarge photodetector comprises applying a voltage between a photocathode and anode that is sufficiently large to form a plasma of a gas in a cavity disposed in an insulator separating the photocathode and anode and creating avalanche breakdown of the plasma, illuminating the photocathode with incident light of photon energy larger than about a work function of the photocathode to eject photoelectrons into the plasma, and detecting an avalanche of the photoelectrons.
The method may further comprise detecting the arrival of photons (the incident light) at the detector by observing the avalanche breakdown by detecting an increase in light emission from the photodetector or detecting the incident light by detecting an increase in current flowing in the photodetector. The method may comprise forming a plasma from gas in a cavity disposed in the photocathode that extends from the cavity in the insulator, illuminating a semiconductor photocathode with the incoming light from the optical source to be detected, illuminating the photocathode with the source light through an optically transmissive material that seals the cavity, illuminating tapered sidewalls of the cavity in the photocathode, or forming the plasma by applying a voltage between the photocathode and the anode.
In an array of photodetectors, the method may further comprise forming plasmas of different gases in different cavities for different photodetectors in the array, illuminating the photodetectors in the array of photodetectors such that a first set of photocathodes in the array are illuminated with light of photon energy larger than about a work function of the first set of photocathodes and illuminating a second set of photocathodes in the array with light of photon energy smaller than about the work function of the second set of photocathodes, thereby ejecting photoelectrons into plasmas associated with the first set of photocathodes but not the second set of photocathodes.
Another embodiment is a detector system that comprises an emission source and a microdischarge photodetector. The photodetector comprises a photocathode, an anode, an insulator disposed between the photocathode and anode, and a gas disposed in a cavity formed in the insulator. The photodetector is disposed in the system to detect light from the emission source that is incident on the photodetector and generate a signal that is proportional to an amount of the incident light falling on the photodetector.
The detector system may further comprise a communication device that receives the signal from the photodetector and notifies an individual of the signal. This communication device may be a display that displays results to an observer.
In the detector system, the cavity may extend into the photocathode, which may be a semiconductor. The cavity may be tapered. Additionally, the insulator may comprise a plurality of dielectric layers, at least two of which may have different dielectric constants. An optically transmissive material may seal the cavity. A thickness of the photocathode may be between about one and five absorption lengths of the incident light.
The detector system may further comprise a substrate that is a substantially different material from the photocathode. A surface of the photocathode may be coated with a material having a higher secondary electron emission coefficient than the photocathode. The anode may comprise an electrically conducting screen. An operating voltage of the photodetector may be at most about 120 V.
The detector system may be configured to supply the signal when a voltage is applied between the photocathode and anode that is sufficiently large to form a plasma of the gas and the photocathode is illuminated with the incident light, which has a photon energy larger than about a work function of the photocathode to thereby eject photoelectrons into the plasma, and the detector is configured to detect an avalanche of the photoelectrons. This signal may be proportional to an increase in current flowing in the photodetector or light emission from the photodetector.
The detector system may further comprise an array of the photodetectors. At least one cavity in the array may contain a different gas from another cavity in the array, at least one photocathode in the array may have a different work function from another photocathode in the array, at least one photocathode in the array may be coated with a material having a higher secondary electron emission coefficient than the at least one photocathode, or he array may be configured such that at least one photodetector in the array is operable independently from another photodetector in the array.